Code

Group group = new Group();  
group.setId(1);  
group.setName("Ke");  
  
User user1 = new User();  
user1.setId(2);  
user1.setName("Liu");

User user2 = new User();  
user2.setId(3);  
user2.setName("Yue");  
group.getList().add(user1);  
group.getList().add(user2);  
  
Map<Integer, Object> map = new HashMap<Integer,Object>();  
map.put(1, "No.1");  
map.put(2, "No.2");  
map.put(3, group.getList());  
  
String jsonString = JSON.toJSONString(map);  
System.out.println(jsonString);

Output

{1:"No.1",2:"No.2",3:[{"id":2,"name":"Liu"},{"id":3,"name":"Yue"}]}